Method and Apparatus for Timed Measurement of the Voltage Across a Device in the Charging Circuit of a Piezoelectric Element
The present invention concerns a method for timed measurement of the voltage across a device in the charging circuit of a piezoelectric element.
The present piezoelectric elements being considered in more detail are, in particular but not exclusively, piezoelectric elements used as actuators. Piezoelectric elements can be used for such purposes because, as is known, they possess the property of contracting or expanding as a function of a voltage applied thereto. The practical implementation of actuators using piezoelectric elements is advantageous in particular if the actuator in question must perform rapid and/or frequent movements.
The use of piezoelectric elements as actuators proves to be advantageous, inter alia, in fuel injection nozzles for internal combustion engines. See both references EP 0 371 469 B1 and EP 0 379 182 B1 regarding the usability of piezoelectric elements as injection valve actuators. Such piezoelectric elements are charged to a specific, generally working point-dependent, voltage. The piezoelectric elements experience a longitudinal expansion that is used to control the opening and closing of the Injection valves. By appropriately charging and discharging the piezoelectric elements, a desired injection operation or injection profile may be obtained.
FIG. 7 is a schematic representation of a fuel injection system using a piezoelectric element 2010 as an actuator. Referring to FIG. 7, the piezoelectric element 2010 is electrically energized to expand and contract in response to a given activation voltage. The piezoelectric element 2010 is coupled to a piston 2015. In the expanded state, the piezoelectric element 2010 causes the piston 2015 to protrude into a hydraulic adapter 2020 which contains a hydraulic fluid, for example fuel. As a result of the piezoelectric element""s expansion, a double acting control valve 2025 is hydraulically pushed away from hydraulic adapter 2020 and the valve plug 2035 is extended away from a first closed position 2040. The combination of double acting control valve 2025 and hollow bore 2050 is often referred to as double acting, double seat valve for the reason that when piezoelectric element 2010 is in an unexcited state, the double acting control valve 2025 rests in its first closed position 2040. On the other hand, when the piezoelectric element 2010 is fully extended, it rests in its second closed position 2030. The later position of valve plug 2035 is schematically represented with ghost lines in FIG. 7.
The fuel injection system comprises an injection needle 2070 allowing for injection of fuel from a pressurized fuel supply line 2060 into the cylinder (not shown). When the piezoelectric element 2010 Is unexcited or when it is fully extended, the double acting control valve 2025 rests respectively in its first closed position 2040 or in its second closed position 2030. In either case, the hydraulic rail pressure maintains injection needle 2070 at a closed position. Thus, the fuel mixture does not enter into the cylinder (not shown). Conversely, when the piezoelectric element 2010 is excited such that double acting control valve 2025 is in the so-called mid-position with respect to the hollow bore 2050, then there is a pressure drop in the pressurized fuel supply line 2060. This pressure drop results in a pressure differential in the pressurized fuel supply line 2060 between the top and the bottom of the injection needle 2070 so that the injection needle 2070 is lifted allowing for fuel injection into the cylinder (not shown).
A more detailed description of a corresponding system can be found at German patent application Nos. DE 197 42 073 A1 and DE 1976 29 844 A1, which are hereby incorporated by reference herein in their entirety. These patent applications disclose piezoelectric elements with double acting, double seat valves for controlling injection needles in a fuel injection system.
In order to achieve precise fuel injection volumes, high accuracy in the degree of longitudinal expansion of the piezoelectric element is important, and, hence, a high accuracy in the charge voltage level is important. Aging phenomena and temperature may have marked effects on the longitudinal expansion, or stroke, and capacitance of a piezoelectric element. A desired stroke may require different charge voltages, depending on the age and/or temperature of the actuator element. In order to ensure a desired stroke of the actuator and especially of an associated valve, the charge voltage must be accordingly regulated. It is therefore important to be able to measure the voltage across a piezoelectric element in a timely and accurate fashion. It may also be important to be able to measure the voltage across a buffer capacitor in the charging circuit of a piezoelectric element for diagnostic purposes.
An object of the invention is to measure the voltage values of devices in the charging circuit of a piezoelectric element in a timely and accurate fashion using a simple measurement and timing concept. The capacitance and energy loss or the power dissipation factor of the actuator may be determined. It is thereby possible to compensate for actuator aging phenomena and accordingly regulate the actuator reference voltage. The buffer capacitor and associated circuitry may also be diagnosed.
The present invention provides a method in accordance with the preamble of claim 1, i.e., a method for timed measurement of the voltage across a device in the charging circuit of at least one piezoelectric element. The voltage across the device is sensed, and the sensed voltage is read at a predefined time in synchronization with an injection operation of the at least one piezoelectric element.
The present invention also provides an apparatus for timed measurement of the voltage across a device in the charging circuit of at least one piezoelectric element. A voltage measuring device is provided. The voltage measuring device senses a voltage across the device, and the voltage measuring device reads the sensed voltage at least one predefined time in synchronization with an injection operation of the at least one piezoelectric actuator.
The device whose voltage is measured may be the at least one piezoelectric element itself of the buffer capacitor of the charging circuit.
The present invention also provides an apparatus for timed measuring of a voltage across a first bank of piezoelectric elements arranged in parallel and a voltage across a second bank of piezoelectric elements arranged in parallel. A voltage measuring device is provided. The voltage measuring device senses the voltages, and the voltage measuring device reads the sensed voltages at a predefined time in synchronization with an injection operation of at least one of the piezoelectric elements.
The present invention employs a voltage measurement triggered in actuator-specific fashion in synchronization with the injection operation. Control or correction of the actuator voltage, as well as diagnosis of the buffer capacitor, is thereby enabled. The desired actuator stroke can be achieved with greater accuracy than before, thus yielding more accurate injection.